Known SAW components always include a piezoelectric substrate, on the surface of which is provided an acoustic track with component structures arranged therein, e.g., interdigital transducers and reflectors. An electrical signal is converted into an acoustic wave and vice versa in the interdigital transducers.
The acoustic wave propagates in accordance with the mostly periodic arrangement of the electrode fingers of the transducer primarily in both longitudinal directions. Transducers are also known that are used in recursive filters and that preferably radiate an excited acoustic wave only in one longitudinal direction. During propagation of the acoustic wave, diffraction losses occur in a marginal area of the transducer due to radiation of a portion of the surface acoustic wave in the transverse direction.
In most piezoelectric substrates with normal dispersion, e.g., quartz, LiNbO3 and YZ, the velocity of propagation of the excited surface acoustic waves in the acoustic track (SAW track) is reduced, compared to a free substrate surface, by metallization of the substrate surface. Thus, one SAW track or several SAW tracks connected electrically to one another cooperate with adjacent outer areas of the substrate surface in the transversal direction as a waveguide. In the waveguide, transversal wave modes, basic mode, and higher modes, are excitable. The higher modes frequently contribute to unwanted resonances in the stop band or upper pass band area of the SAW component and thus wastefully consume some energy of the wave. These resonances lead to unwanted ripples in the pass band area and furthermore are manifested in increased insertion loss of the component and interfering peaks in the frequency response of the group delay. The filter properties of the component suffer from this.
In previously known methods for suppressing interfering transversal modes, there have been attempts to adapt the transversal excitation profile of an electroacoustic transducer so that feeding of the electrical signal to the greatest extent possible results only in the transversal acoustic basic mode.
For instance, it is possible to vary the transversal length of an overlapping area of two adjacently arranged electrode fingers of an exciting finger pair in the acoustic track in the longitudinal direction so that feeding of the electrical signal is improved in the transversal basic mode. This method is based on overlap weighting and is known, for example, from the publication by W. Tanski, Proc. 1979 IEEE Ultrasonic Symposium, pages 815-823.
Alternatively, while maintaining the distance between opposing busbars of two electrodes of a transducer, it is possible to increase the length of the inactive electrode fingers, which are also called stubs and which oppose the exciting electrode fingers in the transversal direction, and at the same time to correspondingly decrease the length of the overlapping area of an exciting finger pair. The excitation of higher transversal wave modes can only be avoided to a certain degree in this manner.
Another known method for suppressing higher transversal modes and/or for adapting the excitation profile of a transducer to the shape of the transversal basic mode is known, for example, from DE 196 38 398 C2. An acoustic track is divided into several partial tracks, whereby all partial tracks contribute to exciting the acoustic wave. Given N waveguide modes to be suppressed, the acoustic track is divided into N partial tracks, whereby the excitation profile can be adapted to the form of the transversal basic mode by adjusting the track widths and/or the sign of the excitation in each of the partial tracks so that the higher transversal modes are suppressed. A disadvantage of this method is that dividing the track depends on the exact number of waveguide modes to be suppressed and therefore the design of the component is also complex.